1. Field of the Invention
The present invention relates to a loudspeaker diaphragm and an electrodynamic loudspeaker, and more particularly to those of an elongated shape being shorter in the minor-axis direction than in the major-axis direction (e.g., a rectangular shape and an elongated circular shape (including an elliptical shape and a racetrack shape)), which have a desirable sound reproducing capability and are not significantly influenced by divided vibrations of an elongated loudspeaker diaphragm, and which are suitably installed in devices such as display devices.
2. Description of the Related Art
With an acoustic device, such as a display device, in which a loudspeaker is installed for reproducing sound, there is a demand for reducing the amount of space required for installing the loudspeaker. Particularly, electrodynamic loudspeakers of an elongated shape (e.g., a rectangular shape and an elongated circular shape (including an elliptical shape and a racetrack shape)) have disadvantages in the sound reproducing capability for various reasons, such as: the diaphragm area is limited in the minor-axis direction; there is a significant influence of divided vibrations that are characteristic of a loudspeaker diaphragm of an elongated shape, whereby it is difficult to obtain a flat frequency response; and when a foam resin material, or the like, is employed in order to achieve both a high rigidity and a light weight of the loudspeaker diaphragm, it will not be possible to set the highest reproducible frequency to be high. Therefore, various loudspeaker diaphragms, loudspeaker magnetic circuits and electrodynamic loudspeakers using the same have been proposed in the art in order to solve these problems in the prior art.
For example, Japanese Laid-Open Patent Publication No. 58-92198 discloses an electrodynamic loudspeaker, including a voice coil bobbin that is in contact simultaneously with two nodes in different ones of free vibration modes of a rectangular planar diaphragm, with a voice coil provided around the bobbin reciprocating in a gap of a magnetic circuit for driving the rectangular planar diaphragm in a node-driving method, wherein a peripheral portion of the rectangular planar diaphragm is connected to a frame via an asymmetric edge member having a greater width on a short side farther away from a driving section for driving the voice coil than on another short side closer to the driving section for driving the voice coil.
Japanese Utility Model Publication for Opposition No. 40-32164 discloses a loudspeaker having a rectangular planar diaphragm of a foam resin, wherein sides along the major axis of the planar diaphragm are fixed to a frame with other sides along the minor axis thereof being free edges, thereby stabilizing the operation. Japanese Laid-Open Patent Publication No. 2001-285991 discloses a loudspeaker in which a generally arc-shaped elastic resin member is laid on the edge at each corner formed by the frame and subjected to a heating process, providing an elastic resin layer in the vicinity of each corner portion of the edge, which often deforms when driving the loudspeaker, thereby absorbing and reducing the distortion due to bending, deformation, etc., of the edge, and thus preventing the consequent deterioration of the sound quality.
Japanese Laid-Open Utility Model Publication No. 50-61834 discloses a loudspeaker diaphragm obtained by laminating together a thermoplastic film foam, and a body that is light-weighted and rigid, with no vibrating substrate being used in the edge portion. Japanese Laid-Open Patent Publication No. 57-132500 discloses a loudspeaker diaphragm formed by laminating a thermoplastic resin film on one side or on both sides of an unexpanded thermofoaming resin sheet and subjecting the laminate to a heat process in a mold of a diaphragm shape to plasticize the thermoplastic resin film and to expand the thermofoaming resin sheet, thus forming the laminate into the diaphragm shape, with a peripheral extension of the thermoplastic resin film being formed as the edge.
Japanese Laid-Open Patent Publication No. 2007-180910, filed by the applicant hereof, discloses a loudspeaker diaphragm, including an edge junction portion provided along an outer periphery of the diaphragm and joined with an edge, and a diaphragm portion provided on an inner periphery side of the edge junction portion and joined with a voice coil, wherein the diaphragm portion has a virtual reference plane defined as a plane containing the edge junction portion, and the diaphragm portion has a wavy shape including a plurality of node lines each defined as a line along which the diaphragm portion intersects with the virtual reference plane and a plurality of ridge lines each defined as a line that connects together points where the distance from the virtual reference plane takes a local maximum, and wherein the plurality of node lines are not parallel to, and do not cross, a major axis of the diaphragm, with one or more of the node lines crossing a minor axis of the diaphragm. Forming a loudspeaker diaphragm in such a wavy shape improves the strength of the elongated diaphragm in the longitudinal direction, i.e., the major-axis direction, whereby it is possible to shift the resonance frequency at which divided resonance occurs in the major-axis direction to a higher frequency. Moreover, the intervals between ridges/troughs of the wavy shape in the minor-axis direction gradually change in the major-axis direction, whereby it is possible to break up the resonance frequency of the divided resonance, which is likely to occur in the major-axis direction. As a result, by using an elongated diaphragm having such a wavy shape, it is possible to realize a loudspeaker having a desirable sound reproduction quality with few peaks and dips in the frequency response.
In order to realize a desirable sound reproduction with an acoustic device such as a display device, it is preferred that an electrodynamic loudspeaker having a sound reproduction quality as good as that of Japanese Laid-Open Patent Publication No. 2007-180910 is installed in the acoustic device. However, when one employs a loudspeaker diaphragm that is formed by using a foam resin to reduce the weight thereof for the purpose of improving the reproduced sound pressure level, the influence of divided vibrations in the major-axis direction becomes pronounced, thereby causing various problems. For example, it may become difficult to realize a flat frequency response, and it may not be possible to increase the highest reproducible frequency. Where the loudspeaker diaphragm is supported with its end portion being fixed, the symmetry between the forward and backward amplitudes may become poor, thus deteriorating the distortion characteristics. In the context of the expression “forward and backward amplitudes” of the speaker diaphragm vibration as used herein, the side on which the voice coil, the damper and the magnetic circuit are attached is the rear side or the backward direction, and the side on which the diaphragm is exposed is the front side or the forward direction.